Such a brushless motor is generally comprised of a stator with a plurality of coils wound around an iron core, a rotor rotatably disposed inside the stator, and a housing containing the stator and the rotor. The housing is provided with a plurality of air holes so that heat generated from the rotor and the stator is released through the air holes.
A brushless motor having a cylindrical housing closed at its opposite ends with covers formed with air holes, and a rotor provided with a fan, is proposed in, e.g., Japanese Patent Laid-Open Publication No. HEI-1-110035. This brushless motor will be described in detail with reference to FIG. 15 hereof.
A brushless motor 300 shown in FIG. 15 includes a stator 303 formed by winding coils 302 around an iron core 301, a rotor 307 provided inside the stator 303 and consisting of a yoke 305 mounted on a rotor shaft 304 and a permanent magnet 306 mounted on the periphery of the yoke 305, and a cylindrical housing 308 containing the stator 303 and the rotor 307 together.
Opposite ends of the housing 308 are closed by covers 309, 309. One cover 309 is provided with a plurality of air inlet openings 310, and the other cover 309 is provided with a plurality of air outlet openings 311.
A fan 312 is mounted on the rotor shaft 304 near the air inlet openings 310. The fan 312 is rotated by rotation of the rotor shaft 304, drawing air into the housing 308 through the air inlet openings 310.
Air drawn into the housing 308 cools the heated stator 303, and then is discharged from the air outlet openings 311.
In the above brushless motor 300, however, a central part of the rotor 307 is not directly cooled, causing the problem that heat persists in the central part of the rotor 307.
Also, in the above brushless motor 300, the cooling fan 312 is attached on the rotor shaft 304, thus increasing the moment of inertia of the rotor 307. Therefore, it is required to increase the rigidity of the housing 308 and the covers 309, 309 for supporting the rotor 307, disadvantageously leading to a weight increase of the brushless motor 300.
There is thus a desire for an art which allows heat in a central part of a rotor to be released, and also allows an electric motor to be reduced in weight.